LM/LK ascent question

Pat Flannery writes:

The astronauts take star sightings and feed the info into the Inertial
Measuring Unit to get everything aligned as to the LM's position and
orientation on the Moon's surface: http://history.nasa.gov/alsj/aot.htm
So they can indeed update their guidance system to compensate for gyro
drift while on the lunar surface.

I have read Digital Apollo, & the small book on the AGC, and I am
still astonished as to how such a wimpy box could do such complex
calculations; the map data alone must be most of the ROM space.
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